Acrylic compositions and articles made from them are well known for their clarity, lack of color, surface gloss, excellent processability, good scratch resistance and resistance to UV degradation. They are also well known for their low-impact strength or brittleness. It has become standard practice to incorporate toughening agents or impact modifiers in acrylic formulations to improve their mechanical properties.
One type of polymeric impact modifier is sequentially polymerized acrylic compositions comprising a core layer, one or more intermediate layers, and an outer-stage layer, although there can be some intermingling of the “layers”. As described in U.S. Pat. No. 3,661,994, these are known as “core/shell” polymers, where either the core layer or an intermediate layer is made of a rubber polymer, and the outer-stage layer, also known as the shell layer, consists of a glassy polymer that is compatible with the acrylic polymer matrix. In the case of a rubbery core layer, the impact modifiers typically suffer from relatively low modulus and relatively poor UV resistance, which is detrimental to the impact modified acrylic formulation. As described in U.S. Pat. No. 4,521,568 and US2003/0216510, core/shell polymers consisting of a hard glassy core layer, and one or more rubber intermediate layers are more desirable to impact modify an acrylic polymer while retaining high modulus and good UV resistance.
Impact modified acrylic resin formulations are well known. A problem with impact modified acrylic resin formulations is that the addition of the core/shell modifiers impairs the melt processability of the polymer matrix by increasing the overall melt viscosity. One known way to improve the melt processability of an impact modified acrylic resin formulation is to decrease the molecular weight of the acrylic polymer matrix, either by using a lower molecular weight acrylic polymer or by incorporating low molecular weight acrylic polymer additives. However, this approach typically degrades other physical properties of the final resin formulation, such as its resistance to chemical attack. There is a desire to improve the melt processibility of impact modified acrylic resin formulations, with little or no degradation of other physical properties.
Surprisingly it has been found that the addition of specific low melt viscosity polymers that are miscible, semi-miscible, or compatible with acrylic polymers, increase the melt processibility of an impact modified acrylic resin, while retaining or improving other physical properties, such as resistance to chemical attack. Polylactic acid is one example of such low melt viscosity polymers. Hard-core core/shell impact modifiers are the preferred acrylic impact modifier.
Polylactic acid (PLA) has been modified with acrylic polymers to improve the properties of the polylactic acid. US 2007/0179218 describes a blend of PLA with 0.1 to 15% acrylic copolymer, and optionally with a core/shell impact modifier.
A PLA matrix has also been modified with acrylic polymer modifiers to improve the heat resistance, impact resistance, and mechanical properties of PLA. US 2007/0276090, and US 2009/0018237 describe a blend of PLA matrix, acrylic polymer and a core/shell impact modifier having a soft core layer. Unfortunately soft-core core/shell impact modifiers suffer from relatively low modulus and relatively poor UV resistance, and therefore their use as an impact modifier significantly decreases the overall modulus and weatherability of the product, much more than that of a hard-core core/shell impact modifier.